This invention relates to a tilt servo circuit for controlling an optical axis of an optical pickup perpendicularly to the surface of an optical disc such as a Laser Vison Disc (LV) or a Compact Disc (CD) played back by an optical disc playback device and, more particularly, to such tilt servo circuit capable of detecting inclination of the disc surface without provision of a tilt sensor.
An optical disc playback device for a LV or CD is so designed that an optical axis of an optical pickup is maintained perpendicular to the normal disc surface. There however actually occur an upward warp 10' or a downward warp 10" with respect to the normal disc surface as shown in FIG. 2 due to heat and gravity. If optical axis 1 of an optical pickup 12 is disposed upwardly in a fixed manner with respect to a disc 10" having a large downward warp as shown in FIG. 3, vertical displacement of the disc surface occurring as the optical pickup 12 slides in a radial direction can be absorbed by focus servo at positions .circle.a and .circle.b to focus laser beam 14 on the disc record surface. At position .circle.c in the vicinity of outer periphery of the disc, however, the amount of vertical displacement of the disc surface can no longer be absorbed by focus servo with a result that laser beam 14 cannot be focused accurately on the disc record surface so that a signal cannot be reproduced. A disc 10' having a large upward warp has the same problem.
Further, when the laser beam 14 is perpendicular to the disc 10 as shown in FIG. 6, the optical axis 1 of the laser beam 14 makes a straight advance through a cover glass 16 and is focused on the record surface 18. In this case, distribution of intensity of the laser spot is shown in FIG. 7. In this figure, intensity of primary interference rings occurring in the vicinity of tracks T-1 and T+1 adjacent to a target track To is relatively small and the intensity in an outer peripheral side portion a and the intensity in an inner peripheral side portion b are equal to each other. If, however, the disc 10 is warped to incline with respect to the optical axis 1 as shown in FIG. 8, the laser beam 14 is refracted and thereby is caused to have aberration. This causes the intensity of the primary interference ring in the outer peripheral side portion a to become stronger as shown in FIG. 9 with a result that a signal on the adjacent track T+1 leaks in a larger amount than a signal on the adjacent track T-1.
Such crosstalk brings about so-called "ghost" in a reproduced picture in a video disc and increases jitter and error rate in a Compact Disc.
Such inclination of the laser beam with respect to the perpendicular direction to the disc surface occurs not only when there is warp in the disc but also when there is inclination in a turntable or when there is an optical axis angle error during manufacturing of the optical pickup.
For coping with this problem, there is a prior art tilt control device for controlling the angle of the optical pickup so as to maintain the optical axis of the laser beam constantly perpendicularly to the disc surface. According to this tilt control, as shown in FIG. 4, a guide shaft 20 along which the optical pickup 12 slides in radial direction is pivoted about a pivot 22 in a plane which passes through the center of the disc 10 and is perpendicular to the surface of the disc 10. There is another type of prior art tilt control device according to which, as shown in FIG. 5, a head base 24 which slides along a guide shaft 20 comprises a subbase 26 on which the optical pickup 12 is mounted and this subbase 26 is povoted about a pivot 28 in a plane which passes through the center of the disc 10 and is perpendicular to the surface of the disc 10.
In performing tilt control by such prior art devices, as shown in FIG. 10 for example, the subbase 26 is supported on the support 30 in a head base (not shown) slidable along a guide shaft in such a manner that the subbase 26 is pivotable about the pivot 28. A tilt sensor 32 is provided in the vicinity of the optical pickup 12 on the subbase 26 for detecting inclination of the optical axis of laser beam with respect to the perpendicular direction to the disc surface.
The tilt sensor 32 comprises a light emitting element 34 and ligth receiving elements 36 and 38. Light emitted from the light emitting element 34 is reflected on the disc surface and received by the light receiving elements 36 and 38. If there is no inclination (tilt error) of the optical axis 1 of the laser beam with respect to the perpendicular direction to the disc surface, the reflected light is received in the equal amount by the light receiving elements 36 and 38 whereas if there is inclination, the reflected light is received in a larger amount by one of the light receiving elements than by the other. Accordingly, by applying a difference voltage between the outputs of these light receiving elements 36 and 38 to a tilt control motor or the like device to control the pivoting angle of the subbase 26, tilt control is effected and the inclination of the optical axis 1 of laser beam thereby is corrected.
The prior art tilt control devices require the tilt sensor 32 provided exclusively for detecting tilt error which brings about increase in the manufacturing cost. Besides, angle of mounting of the light emitting element 34 of the tilt sensor 32 must be adjusted accurately so that the optical axis of the light emitting element 34 becomes parallel to the optical axis 1 of laser beam of the optical pickup 12 and adjustment of offset voltage due to irregularity in characteristics of the light receiving elements 36 and 38 becomes necessary. The adjustment of the tilt sensor 32 therefore is troublesome and time-consuming. Since, further, there is some difference between position of radiation by the tilt sensor 32 and position of radiation by the optical pickup 12, an error remains due to existence of a border with a mirror portion on the disc surface, scratch or warp on the disc surface and this remaining error causes an unnecessary tilt operation with resulting adverse effect on the subsequent reproduction of a signal from the disc.
It is, therefore, an object of the invention to provide a tilt servo circuit which obviates the tilt sensor which has been used exclusively for tilt control in the prior art tilt control devices.